(1) Field of the Invention
This invention relates to a novel continuous process for production of polyethylene. More particularly, this invention relates to an improved process for production of polyethylene in the presence of a Ziegler-type catalyst at high temperature and high pressure.
Hitherto, polymerization and copolymerization of ethylene have been carried out by the use of a conventional catalyst at a pressure of 200 atmospheres or more and at a temperature of 125.degree. C. or more. In such processes, the resulting reaction mixture is introduced into at least one separator when it comes out of a polymerization reactor. This separator is operated to separate ethylene (and any other monomer) from the polymer produced in the reactor in one or more steps. In general, the pressure in the first separator is selected to be 500 atmospheres or less.
When a Ziegler-type catalyst is used in the polymerization reactor in the conventional processes, polymerization (homo- or co-polymerization) or oligomerization of monomer (or monomers) occurs successively under the above mentioned operating conditions of the separator (e.g., temperature, pressure, residence time, etc.), which results in formation of undesired products such as low-molecular products and waxy products, and the like.
The present invention aims to avoid such undesirable results.
(2) Prior Art
In the low-pressure polymerization of .alpha.-olefins in the presence of a Ziegler-type catalyst, it has been known to terminate the reaction by adding to the reaction mixture a variety of reaction agents such as alcohols, epoxides or alkali metal hydroxides.
Thus, it would be readily considered by those skilled in the art to add a polymerization inhibitor or a catalyst-decomposition agent to the reaction mixture to prevent polymerization or oligomerization in the above mentioned separator.
In industrial plants operated at high pressure and temperature, however, an unreacted monomer or monomers are normally recycled for reuse. If the conventional reactive agent mentioned above in an amount sufficient to deactivate the catalyst should be added at the outlet of the reactor or the inlet of the separator, the reactive agent will also be recycled into the polymerization reactor to retard the polymerization reaction.
In this connection, there is proposed as shown in Japanese Laid-Open Patent Specification No.11182/76 a method for preventing retardation of the recycling polymerization reaction wherein a solid alkali metal or solid salt thereof is used as a reactive agent, and the resulting reaction mixture of the metal salt and the catalyst is caused to remain in the resulting polymer.
The alkali metal or salt thereof is insoluble in a non-polar hydrocarbon solvent which is sometimes used as a polymerization dispersion medium. Thus, it is very difficult to add such an agent to a high-pressure region such as the position just before the outlet valve of the polymerization reactor or the inlet of the separator. Furthermore, the additive agent is solid and thus has poor reactivity with the catalyst. In this case, in order to deactivate the catalyst completely, it is generally necessary to use the additive agent in an amount of 1 or more gram equivalent of the alkali metal contained in the metal salt per the sum of aluminum and titanium in the gram atom contained in the catalyst.